Bear in mind all these things are only rough guides. For example, one of the seat size calculators told me I needed a 17" - fair enough but since I ride in a medium-depth tree (17.5 GP) I'd need a flatter tree shape to fit in the smaller seat.
And also your femur length is a very minor consideration when choosing seat size compared with your height, build and the size of your horse. But some other posters here don't agree.
Yes! These are guides to give you a ballpark idea of where to start. They're not exact my any means. The seat size calculators tell me I need an 18", and I usually use a 17.5 .
Unclearthur, the explanation you gave a few days ago of why the seat size actually does reflect the butt-size rather than the femur length was excellent. I've always heard that it's all about the femur length, and your insight made a lot of sense. It does explain why all calculators say I need an 18, and I really use a 17.5 - long legs, flatter butt.
I think much of the reason why we're always told that it's about femur length has to do with the fact that most saddles only come with 1 flap size/length. Ideally, with lots of saddles available and cost no concern, we would size the seat to our butts, then find the correct flap for the leg. Most of us end up trying to find a saddle that will accommodate both as-is (especially since we're also trying to fit the horse - so many variables), so we're often having to go up or down 1/2 a seat size to get a flap that will work for our build. I notice the seat-size calculators measure from the back of the butt to the front of the kneecap, so they seem to be trying to get an estimate based on a combined butt/femur size.
I'm curious what your opinion is on the stirrup-bar location for various build types. It seems like if someone has a larger build, but really short legs, then ideally they would have a seat on the larger size to fit their butt. Do you find that in that case the stirrup bar ends up being too far forward for the legs, resulting in a chair seat? Or does the hip joint on a larger body in a larger seat generally end up being in the correct position above the stirrup bar? Just trying to learn...